In a communication system, a received waveform from a user equipment is a mixture of signal and noise. How to separate the signal and noise power from the received waveform is a fundamental technique for uplink receiver performance.
As an example, FIG. 1 illustrates a channel model for a Wideband Code Division Multiple Access (WCDMA) system. Here, a user equipment (UE) has multiple concurrent channels, such as Sc(t) for a control channel signal and Sd(t) for a data channel signal. While the UE may send many concurrent channels, only two channels are shown for the sake of brevity and simplicity of disclosure. In particular, the UE applies respective orthogonal codes at spreaders 10, 12 for each signal, and a transmitter 14 transmits the multiple signals over a medium such as an air interface. Noise n(t) is an additive channel noise added by transmission over the medium. A base station or NodeB therefore receives the following waveform at its receiver 20:x(t)=Sc(t)+Sd(t)+n(t)
To accurately receive the desired signals, the signal power and noise power are separated from the received waveform in x(t). Signal power and noise power may be defined as:
Noise Power:                E[n2]=function of x(t) for a symbol        
Control Signal Power:                E[Sc2]=function of x(t) for a symbol        
Data Signal Power:                E[Sd2]=function of x(t) for a symbol        
This separation problem is a classic problem in communication theory. FIG. 2 illustrates an existing solution applied in a WCDMA system. This solution applies a correlation function based algorithm. For the sake of brevity and simplicity, FIG. 2 shows this solution applied to the control signal only. As shown, a control channel despreader 30 despreads the received antenna waveform x(t) using the orthogonal code associated with the control channel. Accordingly, the output y(k) from the despreader 30 is the control signal and noise, while the data signal has been canceled. The output symbol is:y(k)=Sc(k)+n(k), where k is a symbol index.
A delay 32 delays the output symbol such that a multiplier 34 multiplies the current and previous outputs symbols to obtain the control signal power. Namely, the control signal power is determined based on correlation function:
                                          E            ⁡                          [                              Sc                2                            ]                                ∼                ⁢                =                              y            ⁡                          (              k              )                                *                      y            ⁡                          (                              k                -                1                            )                                                                      ⁢                  =                                    [                                                Sc                  ⁡                                      (                    k                    )                                                  +                                  n                  ⁡                                      (                    k                    )                                                              ]                        *                          [                                                Sc                  ⁡                                      (                                          k                      -                      1                                        )                                                  +                                  n                  ⁡                                      (                                          k                      -                      1                                        )                                                                                                                      ⁢                  =                                                    Sc                ⁡                                  (                  k                  )                                            ⁢                              Sc                ⁡                                  (                                      k                    -                    1                                    )                                                      +                          {                                                                    Sc                    ⁡                                          (                      k                      )                                                        ⁢                                      n                    ⁡                                          (                                              k                        -                        1                                            )                                                                      +                                                      Sc                    ⁡                                          (                                              k                        -                        1                                            )                                                        ⁢                                      n                    ⁡                                          (                      k                      )                                                                      +                                                      n                    ⁡                                          (                      k                      )                                                        ⁢                                      n                    ⁡                                          (                                              k                        -                        1                                            )                                                                                  }                                                                      ⁢                  =                                                    Sc                ⁡                                  (                  k                  )                                            ⁢                              Sc                ⁡                                  (                                      k                    -                    1                                    )                                                      +                                          o                c                            ⁡                              (                                  Sc                  ,                  n                                )                                                                                    ⁢                  ->                      Sc            2                              where oc(Sc,n)=Sc(k)n(k−1)+Sc(k−1)n(k)+n(k)n(k−1) is called a correlation remainder.
In FIG. 2, the noise power is also calculated based on the correlation function. Here, a power determination unit 36 determines the power of the output symbol and then a combiner 38 subtracts the control signal power output by the multiplier 34 from the output symbol power as shown in the expression below to obtain the noise power:
                                          E            ⁡                          [                              n                2                            ]                                ∼                ⁢                =                                            y              ⁡                              (                k                )                                      2                    -                                    y              ⁡                              (                k                )                                      ⁢                          y              ⁡                              (                                  k                  -                  1                                )                                                                                    ⁢                  =                                                    [                                                      Sc                    ⁡                                          (                      k                      )                                                        +                                      n                    ⁡                                          (                      k                      )                                                                      ]                            2                        -                          [                                                Sc                  2                                +                                                      o                    c                                    ⁡                                      (                                          Sc                      ,                      n                                        )                                                              ]                                                                      ⁢                  =                                    [                                                                    Sc                    ⁡                                          (                      k                      )                                                        2                                +                                                      n                    ⁡                                          (                      k                      )                                                        2                                +                                  2                  ⁢                                      Sc                    ⁡                                          (                      k                      )                                                        ⁢                                      n                    ⁡                                          (                      k                      )                                                                                  ]                        -                          [                                                Sc                  2                                +                                                      o                    c                                    ⁡                                      (                                          Sc                      ,                      n                                        )                                                              ]                                                                      ⁢                  =                                                    n                ⁡                                  (                  k                  )                                            2                        +                                          o                n                            ⁡                              (                                  Sc                  ,                  n                                )                                                                                    ⁢                  ->                      n            2                              where on(Sc, n)=2Sc(k)n(k)]−oc(Sc, n) is also called correlation remainder. A first average unit 40 determines the average noise power of a desired number of symbols, and a second average unit 42 determines the average control signal power over the desired number of symbols.
This process provides meaningful output if the following assumptions stand:
Assumption 1: oc(Sc, n)=0 and on(Sc, n)=0, which means that the correlation remainders are zero.
Assumption 2: Sc(k)Sc(k−1)=Sc2, which implies that we pre-know the signal bits, such as pilot bits.
The assumption 1 means that all the correlation items between signal and noise, noise and noise are zero. The assumption 2 means that all the signal bits are pre-known.
These two assumptions are two of the root causes of performance limitations in the existing solution. The assumption 1 stands only if it has an infinite number of symbols for the average. This implies that the algorithm's adapting speed is slow. The performance will be limited if the number of symbols for the average is too small. In practice, the fast changing nature of the transmission power, the fast closed loop updating speed, and the fast channel fading may result in that only a few symbols are available for the averaging. This is not enough to meet the assumption 1 requirement for a high performance receiver.
As for assumption 2, this assumption requires that the signal bits are pre-known. This further limits application of the existing solution to a pilot signal. Unfortunately, the pilot bits are very limited in communication links, particularly, the uplink.